1. Field of the Invention
The present invention relates to a semiconductor device fabrication substrate having a polycrystalline Si layer sandwiched between monocrystalline Si wafers.
2. Description of the Related Art
Heretofore, gettering treatments to remove harmful impurities include extrinsic gettering which gathers harmful impurities in a treatment-originated deformation layer, an impurity diffusion layer or a polycrystalline Si layer formed at the back of a wafer, as disclosed in, for example, Semiconductor Silicon (1981. pp. 743-755), Solid-State Electronics (Vol. 13, pp. 1401-1406, 1970) and Jpn. J. Appl. Phys., Fall 1988 (30a-t-1), and intrinsic gettering which captures and traps harmful impurities using precipitation of oxygen in the lattice in the wafer as disclosed in, for example, Solid-State Technology (Japanese Version, Vol. 11, p. 39, (1981)), Appl. Phys. Letters (30, 175 (1977)) and Jpn. J. of Appl. Phys. (23, L9 (1984)).
However, semiconductor device fabrication processing tends to involve a lower temperature for the diffusion heat treatment. As the temperature for the diffusion heat treatment gets lower, the diffusion range of harmful impurities, such as heavy metals, becomes smaller. Therefore, the conventional extrinsic gettering that does gettering from the back of a wafer, has a difficulty in gettering in the vicinity of the regions where semiconductor devices are fabricated (hereinafter referred to as "device fabrication regions"), resulting in an insufficient gettering effect. With the use of the intrinsic gettering, gettering near the device fabrication regions is possible by increasing the initial oxygen concentration to produce oxidation induced stacking fault with high concentration and build up the getter sites up to near the wafer surface. Due to poor controllabilities of oxidation induced stacking and of oxygen concentration, however, such stacking fault penetrates through the device fabrication regions, causing leakage.